The present invention relates to a magnetic core available for an RF accelerating cavity for accelerating charged particles and an RF accelerating cavity in which the magnetic core is used.
In recent years, particle accelerators have been widely used not only in the research of nuclear physics, but also in the development of high technologies in medical science, material science, life science, etc. In synchrotrons, an RF cavity for generating an RF voltage is needed for accelerating ions. Usually, an accelerating cavity with a frequency band of several MHz, in which a magnetic member is used in the resonator of the cavity, is used. A high accelerating voltage is required especially when an accelerating cavity is used in high intensity proton accelerators.
As shown in FIG. 5, an RF accelerating cavity in which the magnetic member is loaded has an accelerating cavity 2 in the middle of a cylindrical vacuum duct 1 and magnetic cores 3b and 3c are oppositely loaded around the vacuum duct 1. A coaxial transmission line is composed of the vacuum duct 1 and an external cover 5. When a current is fed from an RF power supply 4, an RF voltage is generated in the accelerating cavity by the resonance between the inductance of the magnetic cores and the capacitance of the accelerating cavity and ion beams are accelerated by the RF voltage.
Further, because the orbiting speed increases with increasing accelerating energy of ion beams, it is necessary to increase the resonant frequency of the accelerating cavity with a lapse of time. Usually, a bias power supply 6 is installed and coils are wound on the magnetic cores, thereby controlling the permeability of the magnetic cores in the external magnetic field formed by the bias current in order to increase the resonant frequency.
An Ni--Zn ferrite has been used in the magnetic core for the RF cavity. Recently it has been proposed to use, as an accelerating cavity, magnetic cores formed with a thin strip of nanocrystalline soft magnetic alloy disclosed in JP-A-6-333717 and JP-B2-2856130, in which fine nanoscale grains with a grain size less than 50 nm are formed with at least 50% of the alloy structure of the strip. These techniques are described in a report of "RF Accelerating cavity" by Yoshii, Seminar on High-Energy Accelerators, OHO96(1996), etc.
The performance of a magnetic core for an accelerating cavity is evaluated by the .mu.'Qf-value in which .mu.', the real part of the complex permeability of the magnetic core at an operation frequency f, and the Q-value are used. An excellent accelerating cavity that operates with a small loss and with high efficiency can be obtained by using a magnetic core in which the .mu.'Qf' value is high. Incidentally, the Q-value is defined by the ratio of the real part .mu.' to the imaginary part .mu." of the complex permeability, .mu.'/.mu." and the higher this value is, the more excellent the performance of the magnetic core will be.
In the accelerating cavity loaded with Ni--Zn ferrite magnetic cores, it has been difficult to increase the accelerating voltage because of low saturation magnetic flux density and the Curie temperature. When high electric power was applied in order to increase the accelerating voltage, magnetic saturation occurred due to heat generation in the ferrite, resulting in a substantial decrease in the .mu.'Qf-value and making the operation of the accelerating cavity unstable. Furthermore, when the above nanocrystalline soft magnetic alloy was used, the .mu.'Qf-value became low because of a low Q-value in the MHz band in which the accelerating cavity operates making it impossible to obtain high performance.